Here's a little problem I've thought about, see if you can solve it.
You like to eat out at a certain restaurant. On the first day you have a 50% chance of eating there. Every day afterwards, if you ate at the restaurant the previous day, the probability of you eating out on that day will be increase by 2%. Otherwise, it will decrease by 2%. Out of 100 days, with proof, what is the expected number of days that you will eat at the restaurant?
If the probability reaches 100%, it will just stay that way.
Good luck!